Optical waveplates are commonly used where light is manipulated in dependence on its state of polarization, such as in liquid crystals (LC) based optical switching and displays. An optical waveplate, which is also known as an optical retarder, is a device that alters the polarization state of a light beam. Common examples of optical waveplates include a half-wave plate that can be used to convert between two orthogonal linear polarizations of light, and a quarter wave plate that can be used to convert between linear and circular polarizations. A typical waveplate may be in the form of a slab of birefringent (BF) material, such as quartz, which has different refraction indices for different polarizations of incident light, and which thickness and orientation is selected to provide a desired retardance, i.e. a desired phase delay between light of orthogonal polarizations at the output of the waveplate.
One application of waveplates is to provide polarization diversity in LC-based light-manipulating devices, where input light of a mixed polarization is first converted into a desired state of polarization, which can then be predictably manipulated using an LC element. The polarization diversity may be accomplished, for example, by first splitting the input light into two orthogonally polarized beams, and then passing one of the two beams through a half-wave plate to equalize its polarization state with that of the second beam. One type of optical switches, such as for example wavelength selective switches (WSS) that are useful as wavelength selective add-drop switches in reconfigurable optical networks, may be configured to operate on a plurality of light beams in parallel. In such multi-port WSSs, input optical beams may be conveniently arranged in a beam stack wherein input and/or output beams are closely aligned next to each other along a direction termed ‘vertical’; the polarization diversity in such switches may require an array of waveplates. Examples of such WSS are described in U.S. Pat. Nos. 7,397,980 and 7,787,720, both of which are incorporated herein by reference.
For example, FIG. 1, illustrates a portion of the front end arrangement of a WSS with vertical beam stacking and polarization diversity. In this arrangement collimated light beams from vertically aligned fiber ports (not shown) are each split into vertically and horizontally polarized sub-beams by a polarization walk-off element (not shown), so as to form a vertical stack input beams 11 of alternating vertical (‘V’) and horizontal (‘H’) polarization. A composite waveplate (WP) 120, which includes alternating regions 121, 122 of birefringent (BF) and non-birefringent (non-BF) material alternating in a vertically stacked configuration corresponding to the arrangement of the input beams 11, may then be used to selectively convert the polarization of vertically-polarized input beams to the horizontal polarization while leaving the horizontally polarized beams unchanged in polarization. Those of the input beams 11 that are horizontally polarized pass through the non-BF plates 122 of the WP 120 without changing its polarization state, while those that are vertically polarized pass through the BF wave plates 121 of the WP 120 that changes their polarization to horizontal, so that all of the output beams 12 after the WP 120 are horizontally polarized. By shifting the WP 120 by one plate width up or down, the polarization state of the output beams 12 may be changed to vertical. The output beams 12 may then be passed through a wavelength dispersive elements, and their constituent wavelengths individually processed using an LC array.
Thus, multi-port wavelength processors with polarization diversity may require a composite waveplate with alternating birefringent and non-birefringent regions. Referring to FIG. 2, which reproduces FIG. 8 of U.S. Pat. No. 7,397,980, such composite waveplates may be constructed from bulk components in a process that includes first bonding together a stack of alternating sheets of birefringent and non-birefringent materials, such as quartz and glass. The front face 225 of the stack is then polished to an optical quality finish, and then cut transversely to the direction of the sheets along line 226. The cut piece may then be attached to a substrate and polished on the cut face to be the required thickness for a λ/2 waveplate at the wavelength of operation λ, after which the substrate may be removed. This process requires mechanical manipulations and precision machining, and is therefore relatively complicated and time consuming.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for providing composite waveplates with alternating birefringent and non-birefringent regions.